The present invention generally relates to a technique for detecting automatically a focus condition of an optical system in camera, microscope, high density optical recording and reproducing apparatus, etc.
Heretofore, there has been developed a still camera comprising an automatic focus detecting device for forming a finely focused image on a predetermined focal plane, i.e. a film plane. For detecting the focus condition automatically, there have been proposed two methods, one method is referred to as an image sharpness detection method and the other method is termed as an image lateral shift detection method. Now the above two methods will be explained.
FIG. 1 is a schematic view showing a general construction of the known focus detection apparatus. In FIG. 1, an image of an object 1 is projected by an imaging optical system 2 onto a light receiving device 3 comprising a number of light receiving regions, i.e. picture cells. Illumination signals supplied from the light receiving device 3 are converted parallelly into digital information by a signal processing circuit 4. The digital information is fed to a central processing unit 5 and is processed therein to generate a focus detection signal representing a focus condition of the optical system 2 with respect to a predetermined focal plane, i.e. a film plane. The focus detection signal thus generated is supplied to a display device 6 to inform a user of the focus condition. The focus detection signal is further supplied via a control circuit 7 to a device 8 such as a motor for driving the optical system 2 to move the optical system in a direction of its optical axis. The control circuit 7 and the driving device 8 may be omitted. In such a case, the optical system 2 may be adjusted manually in accordance with an indication made by the display device 6.
FIG. 2 is a schematic view showing the known focus detection device employing the image-sharpness detection method. The light receiving device 3 comprises a substrate 9 and a pair of arrays 9A and 9B of light receiving elements. These light receiving element arrays 9A and 9B are arranged on respective sides of a predetermined focal plane which is conjugated with the film plane and are separated from the conjugated focal plane by equal distances. In case of applying the focus detection device to a single-lens reflex camera, there is provided a half mirror 12 in a center portion of a quick-return mirror 11 arranged between the imaging lens system 2 and a film 10. A light flux transmitted through the half mirror 12 is reflected by a mirror 13 provided on the rear surface of the quick-return mirror 11 toward a light flux dividing prism 14. The prism 14 comprises a half mirror 15 and a reflection mirror 16 and a half of the incident light flux transmitted through the half mirror 15 is made incident upon the first light receiving element array 9A and the other half of the incident light flux reflected by the half mirror 15 and mirror 16 is made incident upon the second light receiving element array 9B. On the upper surface of the prism 14 is arranged a filter 17 for cutting infrared light.
In the image-sharpness detection method, the central processing unit 5 produces the focal detection signal in accordance with an evaluation function F(x), wherein x is a position of the lens system 2 with respect to a reference position x=0 at which the lens system 2 is focused for an object situating at an infinitive range. Now it is assumed that the number of light receiving elements of each arrays 9A and 9B is equal to N, amounts of output signals from kth and (k+1)th light receiving elements (1.ltoreq.k.ltoreq.N-1) in case that the lens system 2 is in a position x are fx(k) and fx(k+1), and a difference between the output signals is denoted by Mx=.vertline.fx(k)-fx(k+1).vertline.. Then the evaluation function F(x) can be given by a sum of the maximum difference Mx.multidot.1, the second maximum diference Mx.multidot.2 . . . and the (n-b 1)th maximum difference Mx.multidot.(n-1). Thus, the evaluation function F(x) can be given as follows. ##EQU1##
FIG. 3 shows curves representing a relation between the position of the lens system 2 and the evaluation functions F.sub.A (x) and F.sub.B (x) obtained by processing the output signals from the light receiving element arrays 9A and 9B, respectively. In this example, use is made of the lens system 2 having a focal length f=50 mm and an F number of 1.4. FIG. 4 illustrates a curve showing a difference F.sub.B (x)-F.sub.A (x) between the evaluation functions F.sub.A (x) and F.sub.B (x) and this difference is used to determine the focal condition of the lens system 2. When both the evaluation values F.sub.A (x) and F.sub.B (x) are smaller than K in FIG. 3, the evaluation is no more effected, because in such a case the evaluation values do no longer contribute to the focus detection.
As shown in FIG. 4, the difference F.sub.B (x)-F.sub.A (x) has a very steep slope and thus, the focus condition can be detected with an extremely high sensitivity. A possible error might be about .+-.30 .mu.m. However, since the difference F.sub.B (x)-F.sub.A (x) can be obtained within only a very small range in the lens position (x), the focal detection can only be possible within a narrow range near the in-focused position. That is to say, ranges (a) and (b) in which the lens system can be judged to be out of focus forwardly and backwardly, respectively are very narrow. As shown in FIG. 4, the ranges (a) and (b) have a length of about 1.6 mm, while the lens system 2 can be moved over a wide distance of 7 mm. When the lens system 2 is positioned in a range out of the ranges (a) and (b), it is impossible to determine the focus condition and therefore, the user cannot know a direction into which the lens system 2 should be driven or the automatic focus could not work properly.
Next, the focal detection method utilizing the lateral shift of the image will be explained.
FIGS. 5 and 6 are schematic views showing the known focus detection device of the image shift detection method. In this device, the light receiving device 3 comprises an array of optical systems 18-1, 18-2 . . . 18-n such as lenticular lenses and micro lenses, and an array of light receiving elements 19A-1, 19A-2 . . . 19A-n, 19B-1, 19B-2 . . . 19B-n. The optical systems 18 are arranged substantially in the predetermined focal plane conjugated with the plane of the film 10 in such a manner that the exit pupil of the imaging optical system 2 are conjugated with the light receiving elements 19A and 19B with respect to the auxiliary optical systems 18. Thus, each of the auxiliary optical systems 18 serves to divide the image of the exit pupil of the imaging lens system 2 and the divided images are selectively made incident upon a pair of light receiving elements 19A-i and 19B-i. That is to say, in front of each pair of light receiving elements 19A-1, 19B-1; 19A-2, 19B-2; . . . 19A-n, 19B-n there are provided respective auxiliary optical systems 18-1, 18-2 . . . 18-n such as lenticular lenses in such a manner that the pair of light receiving elements are conjugated with respective portions of the exit pupil plane which are divided by a plane perpendicular to the direction of the light receiving element array and containing the optical axis of the imaging lens system 2.
In the focus detection system shown in FIGS. 5 and 6, when at least a part of the image of the object 1 is projected upon the all pairs of light receiving elements 19A-1, 19B-1; 19A-2, 19B-2; . . . 19A-n, 19B-n by means of the imaging lens system 2 and auxiliary optical systems 18-1, 18-2 . . . 18-n, a light flux transmitted through a lower half portion of the imaging lens system 2 is selectively made incident upon the first group of the light receiving elements 19A-1, 19A-2 . . . 19A-n, while a light flux transmitted through an upper half portion of the imaging lens system 2 is selectively made incident upon the second group of the light receiving elements 19B-1, 19B-2 . . . 19B-n. By suitably processing output signals from the light receiving elements, it is possible to detect the focus condition of the imaging lens system 2.
FIG. 7 shows curves y.sup.a and y.sup.b representing the outputs of the first and second light receiving elements 19A and 19B, respectively. It should be noted that in the in-focused condition, these two output curves y.sup.a and y.sup.b become identical with each other, but in the de-focused condition they are shifted from each other. Now, it is assumed that outputs from the kth light receiving elements 19A-k and 19B-k in case that the imaging lens system 2 is at a position x are denoted by y.sub.k.sup.a and y.sub.k.sup.b, respectively. Then, the focus condition of the imaging lens system 2 can be determined by deriving the following evaluation function J(x) by means of the signal processing circuit 4 and central processing unit 5. ##EQU2## A typical evaluation value J(x) is shown in FIG. 8. Although the evaluation value J(x) fluctuates to a small extent due to variation of light receiving elements and quantizing error in the A/D conversion in the signal processing circuit 4, it is possible to determine the focus condition of the imaging lens system 2 over the whole range of the lens position x. That is to say, it is possible to obtain backwardly- and forwardly-defocused regions (a') and (b') having very wide ranges. Of course, the in-focused region (d) including the best focus position (c) can also be obtained. However, in the in-focused region (d), the evaluation value J(x) has a very small inclination and thus the accuracy of the focus detection near the best focus point (c) is low. Usually, there is a relatively large error of .+-.0.1 to .+-.0.2 mm. Therefore, even if one takes a photograph while the in-focused condition is indicated, it is not always possible to obtain a sharply focused image.
In the known lateral image shift detection method, the in-focus region (d) may be narrow when the light intensity of the light receiving elements is increased by making the dimension of the elements large as compared with the maximum dimension of the image of exit pupil of the imaging lens system 2 formed by the auxiliary optical system 18. However, in this case, if the position of the light receiving elements is deviated with respect to the auxiliary optical system, the in-focus region (d) of the evaluation value J(x) might be shifted toward the region (a') or (b') and an erroneous judgement might be effected. In order to avoid such a drawback, in the known focus detection system, the light receiving elements are so constructed that each pair of elements 19A-1, 19B-1; 19A-2, 19B-2; . . . 19A-n, 19B-n are situated within respective images of the exit pupil of the imaging lens system 2 formed by respective auxiliary optical systems 18-1, 18-2 . . . 18-n.
The dimension of the image of the exit pupil of the imaging lens system 2 varies in accordance with the F number of the system 2 and becomes smaller with increasing F number. Therefore, when the dimension of the light receiving elements is determined for the imaging lens system 2 having a small F number, then if the imaging lens system 2 is replaced by another imaging lens system having a larger F number, the above mentioned drawback might appear.
In order to obviate such a drawback, in U.S. patent application Ser. No. 29,498 filed on Apr. 12, 1979, (now U.S. Pat. No. 4,246,476) there has been proposed a focus detection device in which first and second pairs of light receiving elements are provided, the first pair of elements having a dimension substantially equal to the image of the exit pupil of an imaging lens system having the F number of 2.8 and the second pair of elements having such a dimension equal to an image of exit pupil of an imaging lens system of the F number of 4.0. When use is made of an imaging lens system having the F number smaller than 2.8, the first pair of elements is used and when the imaging lens system having the F number smaller than 4.0, but larger than 2.8 is used, the second pair of elements is selected. In such a focus detection device, the selected light receiving element pair has always a dimension smaller than the exit pupil image of the imaging lens system to be used and therefore, the erroneous determination can be effectively prevented. Particularly, in case of using the imaging lens system having the F number equal to 2.8 or 4.0, the image of the exit pupil becomes identical with the dimension of the light receiving element pair, so that the focus condition can be detected with a high sensitivity. However, when use is made of the imaging lens system of F number of 1.4 or 3.5, the dimension of the element pair becomes substantially smaller than the exit pupil image and therefore, the sensitivity is decreased to a great extent.
Now the decrease of the sensitivity will be further explained. The dimension y' of the exit pupil of the imaging lens system 2 formed by the auxiliary optical system 18 can be expressed as follows, ##EQU3## wherein y is a dimension of the exit pupil of the imaging lens system 2, a is a distance between the exit pupil plane of the imaging lens system 2 and the auxiliary optical system 18, and fs is a focal length of the auxiliary optical system 18. If a parallel light flux is made incident upon the imaging lens system 2; ##EQU4## is obtained. Then, the above equation can be rewritten into as follows. ##EQU5## Since the focal length fs of the auxiliary optical system 18 is sufficiently smaller than a(fs/a&lt;&lt;1), ##EQU6## is obtained. This equation means that the dimension y' of the image of the exit pupil of imaging optical system 2 is inversely proportional to the F number of the imaging optical system 2. Therefore, the last equation can be expressed as follows; ##EQU7## wherein C is a constant.
In the above mentioned U.S. patent application, the dimension of the first pair of light receiving elements is set to ##EQU8## and the dimension of the second pair of light receiving elements is set to ##EQU9## When use is made of the imaging lens system 2 having the F number of 1.4, l the dimension of the image of the exit pupil becomes ##EQU10## and this image is received by the first pair of light receiving elements. Then a utility efficiency .eta. of light can be expressed as follows. ##EQU11## This results in that the sensitivity is decreased by four times. Moreover, when the exit pupil image of the imaging lens system having the F number of 3.5 is received by the second pair of light receiving elements, the utility efficiency .eta. becomes as follows; ##EQU12## This means that the sensitivity will be decreased by about 25%.